1. Field of the Invention
The present invention generally relates to a wireless personal area network, and more particularly, to a system and a method for selecting a beacon transmission starting point used by nodes forming a wireless personal area network (WPAN).
2. Description of the Related Art
A wireless personal area network (WPAN) wirelessly interconnects devices, such as computers and peripherals, mobile phones, and home appliances, in a short range of 10 m, supporting communications among the devices, and thus providing various services. The WPAN has been developed to implement networks among the devices with the development of personal mobile digital devices such as laptop computers and personal digital assistants (PDAs). The WPAN has distinctive features, including short range, compactness, low-power, and less interference.
FIG. 1 illustrates a WPAN having a plurality of clusters. A cluster consists of one coordinator and a plurality of nodes. The coordinator manages the nodes in the cluster. The coordinator generates and transmits a beacon to the nodes of the cluster. The nodes of the cluster receive the beacon from the coordinator and acquire a required information from information contained in the received beacon. Some nodes of the cluster of FIG. 1 can transmit the beacon.
FIG. 2 illustrates a beacon transmitted between a coordinator and a plurality of nodes of a cluster, which is described in detail below.
The cluster tree of FIG. 2 consists of one coordinator and the plurality of the nodes. The nodes are classified into nodes capable of transmitting the beacon and nodes incapable of transmitting the beacon. The first and second nodes can transmit the beacon. The third through sixth nodes may or may not transmit the beacon. It is assumed for the sake of clarity that the third to sixth nodes do not transmit the beacon. Frames used by the coordinator and the nodes are also illustrated in FIG. 2. The frame is divided into a beacon transmission period, a data transmission period, and an inactive period. The coordinator and the first and second nodes transmit the beacon in the beacon transmission period, and transmit data in the data transmission period. The coordinator and the first and second nodes do not perform any operation in the inactive period, reducing power consumption.
The first and second nodes receive the beacon from the coordinator; The first node broadcasts the beacon to the third and fourth nodes, and the second node broadcasts the beacon to the fifth and sixth nodes. In general, the nodes randomly select their beacon transmission periods (beacon transmission starting point), as shown in FIG. 2. However, this random selection of the beacon transmission period causes some problems.
Still referring to FIG. 2, the first node receives the beacon from the coordinator in the data transmission period. Accordingly, the first node cannot transmit and receive data to and from the third and fourth nodes during this time. The second node has to receive the beacon from the coordinator even in the inactive period where the second node does not perform unnecessary operations to save power. Albeit not illustrated in FIG. 2, the beacon transmission period of the node and that of the coordinator may overlap since the coordinator and the node randomly select their beacon transmission period.
FIG. 3 illustrates a new node 15 joining the cluster. The new node 15 needs to receive the beacon from the adjacent nodes to communicate with the existing nodes of the cluster. The new node 15 acquires required information using the received beacon.
The new node 15 can receive the beacons from the fourth, fifth, eighth, and ninth nodes. If the fourth and fifth nodes transmit the beacon in a same time slot, the new node 15 receives collided beacons. If the eighth and ninth nodes transmit the beacon in a same time slot, the new node 15 also receives collided beacons. As a result, the new node 15 cannot receive the beacon from any node. This problem may arise when positions of nodes are changed in the cluster.